there is nothing like HA-! It's either A- or HA, as you have it in your equation HA = A- + H+

a) look up definitions of weak and strong acids. b) all you need is pH=pKa+log[A-/HA] c) which form (HA or A-) will be in stomach and which form will be in intestine? Which of these forms will difuse more easily through the plasma mebrane?